1. Field of the Invention
The present invention relates to a hardness test method, a hardness tester, and a computer-readable storage medium storing a program.
2. Description of the Related Art
Conventionally, an indentation hardness tester, such as a Vickers hardness tester, for evaluating and measuring the hardness of a sample on the basis of an indent formed by indenting an indenter, loaded with a predetermined load thereon, on a surface of the sample has been known (see, for example, Japanese Patent Application Laid-Open Publication No. 2005-326169).
Moreover, a test method called nanoindentation (instrumentation indentation test) has been known, which method continuously measures a test force (a force loaded on an indenter) F and an indentation depth (a displacement quantity of an indenter) h to obtain a mechanical property of a material by analyzing an obtained indentation curve (F-h curve) in an indentation process in place of the observation of an indent formed after indentation, (see, for example, Japanese Patent Application Laid-Open Publication No. 2009-47427).
The nanoindentation is effective as an evaluation method of a material the indent observation of which is difficult due to a factor, such as a small size of an indent. Consequently, the nanoindentation is noticed to be suitable for the evaluation of the mechanical properties of a plastic and a thin film material, which are construction materials indispensable for various machines and structures.
For the nanoindentation, International Standard ISO (International Organization for Standardization) 14577 regulates the parameter of hardness called indentation hardness HIT, and the cases of using the indentation hardness HIT for the evaluation of a thin film material and the like have recently increased. The indentation hardness HIT is treated as a value having a correlation with the Vickers hardness.
Here, the analysis method of the indentation hardness HIT that is regulated by the ISO 14577 is shown.
FIG. 7 is a schematic diagram of an F-h curve. The ordinate axis thereof indicates a test force F, and the abscissa axis thereof indicates an indentation depth h.
The indentation hardness HIT is defined by the following formula (1) as a value obtained by dividing the maximum test force (set test force) Fmax by the contact projected area Ap(hc) of a sample of an indenter at the time of the maximum indentation.HIT=Fmax/Ap(hc)  (1)
Then, for example, the contact projected area Ap(hc) is expressed as the following formula (2) from the geometric shape of the Berkovich indenter.Ap(hc)=23.96hc2  (2)
Moreover, hc is called contact depth, and is expressed by the following formula (3) by using the maximum indentation depth hmax, and an intersection point hr of a tangential line of the initial part of a load unloading curve and an indentation depth axis.Hc=hmax−0.75(hmax−hr)  (3)
The aforesaid analysis method of the indentation hardness HIT regulated by the ISO 14577 is a technique proposed by Oliver and Phart, and it was ascertained by their research that there was a correlation between the indentation hardness HIT and the Vickers hardness HV.
However, the samples that they used in their research were ones each having a strong tendency of plastic behavior, such as a metal, and ones each expressing a deformation in which an elastic deformation and a plastic deformation were mixed, i.e. ones clearly expressing elastoplastic behavior, such as fused silica, and they did not examine the materials each having a strong tendency of showing the elastic behavior, such as a rubber material and an amorphous material. In the nanoindentation including also the materials each having a strong tendency of showing the elastic behavior as objects, it is difficult to treat the indentation hardness HIT regulated in the ISO 14577 as being equivalent to the Vickers hardness HV.
For example, FIG. 8 is a diagram showing a relation between the indentation hardness HIT and the Vickers hardness HV of copper, beryllium copper (Cu—Be), tool steel (SK85), fused silica, acrylic resin, polypropylene (PP), and a diamond-like carbon (DLC) film. The ordinate axis thereof indicates the indentation hardness HIT, and the abscissa axis thereof indicates the Vickers hardness HV.
The straight line in FIG. 8 expresses the indentation hardness HIT and the Vickers hardness HV by the following formula (4) by using a coefficient C.HIT=C1·HV  (4)
To put it concretely, the straight line in FIG. 8 expresses the formula (4) when the coefficient C1 is set to 1.25. In addition, as the reasons why the coefficient C1 is not 1 (HIT≠HV), for example, some causes, such as a point of using a projected area, not a surface area, for the calculation of indentation hardness HIT; causes of errors peculiar to the nanoindentation, such as a tip shape of an indenter and a surface detection error; and the like, can be considered. In any event, it can be said that the values of the indentation hardness HIT and the Vickers hardness HV of the samples other than the DLC film have a correlation, although the values are not equal.
On the other hand, the values of the indentation hardness HIT and the Vickers hardness HV of the DLC film greatly deviate from the straight line, and the fact indicates that the treatment of the indentation hardness HIT as being equivalent to the Vickers hardness HV in the regulation of the ISO 14577 has a problem for the DLC film.
That is, the technique for obtaining a value corresponding to the Vickers hardness HV in the nanoindentation is not established in the case where the rubber material, the amorphous material, or the like is used as a sample.